I seem to recall some people getting a length of plastic hose, the clear, heavy duty kind. (The ones we used to make beer funnels with)
Anyway, it was prob between 3/4 and 1 1/2 inches in diameter, and 5 feet long. They capped off one end, coiled it, filled it with gas, and strapped it to their bike. A couple of extra litres at least. Seems like a good plan.
'02 BMW F650GS Dakar, '01 Suzuki Bandit 600S, '93 Yamaha DT50
'93 Dnepr MT-11, '83 Vespa Primavera ET3
, A 20,000km solo ride Canada->Panama->Canada